1. Field of the Invention
The present invention relates generally to drill bits used in the oil and gas industry and more particularly, to drill bits having diamond-impregnated cutting surfaces. Still more particularly, the present invention relates to drag bits in which the diamond particles imbedded in the cutting surface are substantially uniformly coated with matrix to improve diamond retention and wear life.
2. Background Art
An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. When weight is applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone.
Different types of bits work more efficiently against different formation hardnesses. For example, bits containing inserts that are designed to shear the formation frequently drill formations that range from soft to medium hard. These inserts often have polycrystalline diamond compacts (PDC's) as their cutting faces.
Roller cone bits are efficient and effective for drilling through formation materials that are of medium to hard hardness. The mechanism for drilling with a roller cone bit is primarily a crushing and gouging action, in which the inserts of the rotating cones are impacted against the formation material. This action compresses the material beyond its compressive strength and allows the bit to cut through the formation.
For still harder materials, the mechanism for drilling changes from shearing to abrasion. For abrasive drilling, bits having fixed, abrasive elements are preferred. While bits having abrasive polycrystalline diamond cutting elements are known to be effective in some formations, they have been found to be less effective for hard, very abrasive formations such as sandstone. For these hard formations, cutting structures that comprise particulate diamond, or diamond grit, impregnated in a supporting matrix are effective. In the discussion that follows, components of this type are referred to as “diamond impregnated.”
During abrasive drilling with a diamond-impregnated cutting structure, the diamond particles scour or abrade away concentric grooves while the rock formation adjacent the grooves is fractured and removed. As the matrix material around the diamond granules is worn away, the diamonds at the surface eventually fall out and other diamond particles are exposed.
An example of a prior art diamond impregnated drill bit (“impreg bit”) is shown in FIG. 1. The drill bit 10 includes a bit body 12 and a plurality of ribs 14 that are formed in the bit body 12. The ribs 14 are separated by channels 16 that enable drilling fluid to flow between and both clean and cool the ribs 14. The ribs 14 are typically arranged in groups 20 where a gap 18 between groups 20 is typically formed by removing or omitting at least a portion of a rib 14. The gaps 18, which may be referred to as “fluid courses,” are positioned to provide additional flow channels for drilling fluid and to provide a passage for formation cuttings to travel past the drill bit 10 toward the surface of a wellbore (not shown).
Impreg bits are typically made from a solid body of matrix material formed by any one of a number of powder metallurgy processes known in the art. During the powder metallurgy process, abrasive particles and a matrix powder are infiltrated with a molten binder material. Upon cooling, the bit body includes the binder material, matrix material, and the abrasive particles suspended both near and on the surface of the drill bit. The abrasive particles typically include small particles of natural or synthetic diamond. Synthetic diamond used in diamond impregnated drill bits is typically in the form of single crystals. However, thermally stable polycrystalline diamond (TSP) particles may also be used.
In one impreg bit forming process, the shank of the bit is supported in its proper position in the mold cavity along with any other necessary formers, e.g. those used to form holes to receive fluid nozzles. The remainder of the cavity is filled with a charge of tungsten carbide powder. Finally, a binder, and more specifically an infiltrant, typically a nickel brass copper based alloy, is placed on top of the charge of powder. The mold is then heated sufficiently to melt the infiltrant and held at an elevated temperature for a sufficient period to allow it to flow into and bind the powder matrix or matrix and segments. For example, the bit body may be held at an elevated temperature (>1800° F.) for a period on the order of 0.75 to 2.5 hours, depending on the size of the bit body, during the infiltration process.
By this process, a monolithic bit body that incorporates the desired components is formed. One method for forming such a bit structure is disclosed in U.S. Pat. No. 6,394,202 (the '202 patent), which is assigned to the assignee of the present invention and is hereby incorporated by reference.
Referring now to FIG. 2, a drill bit 20 in accordance with the '202 patent comprises a shank 24 and a crown 26. Shank 24 is typically formed of steel and includes a threaded pin 28 for attachment to a drill string. Crown 26 has a cutting face 22 and outer side surface 30. According to one embodiment, crown 26 is formed by infiltrating a mass of tungsten-carbide powder impregnated with synthetic or natural diamond, as described above.
Crown 26 may include various surface features, such as raised ridges 27. Preferably, formers are included during the manufacturing process so that the infiltrated, diamond-impregnated crown includes a plurality of holes or sockets 29 that are sized and shaped to receive a corresponding plurality of diamond-impregnated inserts 10. Once crown 26 is formed, inserts 10 are mounted in the sockets 29 and affixed by any suitable method, such as brazing, adhesive, mechanical means such as interference fit, or the like. As shown in FIG. 2, the sockets can each be substantially perpendicular to the surface of the crown. Alternatively, and as shown in FIG. 2, holes 29 can be inclined with respect to the surface of the crown 26. In this embodiment, the sockets are inclined such that inserts 10 are oriented substantially in the direction of rotation of the bit, so as to enhance cutting.
As a result of the manufacturing technique of the '202 patent, each diamond-impregnated insert is subjected to a total thermal exposure that is significantly reduced as compared to previously known techniques for manufacturing infiltrated diamond-impregnated bits. For example, diamonds imbedded according to methods disclosed in the '202 patent have a total thermal exposure of less than 40 minutes, and more typically less than 20 minutes (and more generally about 5 minutes), above 1500° F. This limited thermal exposure is due to the shortened hot pressing period and the use of the brazing process.
The total thermal exposure of methods disclosed in the '202 patent compares very favorably with the total thermal exposure of at least about 45 minutes, and more typically about 60-120 minutes, at temperatures above 1500° F., that occurs in conventional manufacturing of furnace-infiltrated, diamond-impregnated bits. If diamond-impregnated inserts are affixed to the bit body by adhesive or by mechanical means such as interference fit, the total thermal exposure of the diamonds is even less.
With respect to the diamond material to be incorporated (either as an insert, or on the bit, or both), diamond granules are formed by mixing diamonds with matrix power and binder into a paste. The paste is then extruded into short “sausages” that are rolled and dried into irregular granules. The process for making diamond-impregnated matrix for bit bodies involves hand mixing of matrix powder with diamonds and a binder to make a paste. The paste is then packed into the desired areas of a mold. The resultant irregular diamond distribution has clusters with too many diamonds, while other areas are void of diamonds. The diamond clusters lack sufficient matrix material around them for good diamond retention. The areas void or low in diamond concentration have poor wear properties. Accordingly, the bit or insert may fail prematurely, due to uneven wear. As the motors or turbines powering the bit improve (higher sustained RPM), and as the drilling conditions become more demanding, the durability of diamond-impregnated bits needs to improve. What is still needed, therefore, are techniques for improving the diamond distribution in impregnated cutting structures.